Ozgrade3,. . . .Extra Tanks
Requirements used to be notified in an AWI (Airworthiness Instruction). The system design requirements differ depending on single or twin. However, it is not a problem to engineer a removable system to provide for occasional use. Either way, either you pick up an OS STC acceptable to CASA or the locally designed installation would require the normal CAR35 engineering design approval, LAME involvement, logbook certifications, and AFM supplement(s) and probably an Australian STC.
So far as the physical aspects of the design are concerned, it is more usual to treat the additional fuel as an auxiliary tank installation where the fuel is transferred to a main tank and then to the engine. This greatly reduces the ground and flight test demonstration requirements. Typically, the aux tank line will be Tee-d into the main tank to suit the basic installation and the Tee assembly blanked when the aux tank system is removed.
Overweight ferry operation, likewise, used to be an AWI procedure and has been quite routine for many, many years.
I would suspect that the present procedural requirements involve an Australian STC. While in the past I have been involved with a number of such fuel mods, I have not done any in recent years, so I may not be quite up to speed on the present procedures. If your enquiry is other than an idle interest, I suggest that you try CASA on 131 757 and ask for Steve Hampson in the Sydney office for guidance. Steve is one of the Airworthiness Engineers up there (unless he has moved since I last dealt with the office), a good lad, and will give you a heads up on current requirements.
So far as insurance is concerned, I would be surprised if there would be a problem as the mod process is an approved animal and ought to be viewed no differently to any other mod. However, the best way to check is to refer the question to your insurance agent.
GotTheTshirt,
You have the basic idea .. but the detail and procedure is the devil .. not quite a simple matter of just bolting it on ...